The present invention relates to parking braking systems for vehicles having either conventional or electronically controlled braking systems (EBS).
The accepted approach to parking brakes in vehicles fitted with traditional pneumatic braking systems is to have a connection, either pneumatically, hydraulically or by a cable, between a manually operated actuating device, such as a lever or valve, and some form of braking device at the individual wheels. In heavy vehicles having pneumatically operated brakes, for example, this usually involves releasing the air supply to brake actuators at the wheels to enable the brakes to be applied by strong springs whose force is normally opposed by the air supply in non parking braking modes.
This accepted approach to parking brakes in vehicles fitted with EBS uses spring brake actuators as shown diagrammatically in FIG. 1 of the accompanying drawings. In this system, a hand-operated valve 10 is used, via a relay valve 11, to allow the parking brake to be applied. The valve 10 operates on an inverse air principle in that it is arranged to release air pressure to allow the spring force of respective spring brake actuators 12 at each wheel to be applied. A suitable parking brake reservoir 14 is required to store the pressurised air for use within the system. Where the system is used with a vehicle having a trailer, a separate relay valve (not shown) is required to allow selective operation of the trailer brakes. When the driver operates the hand valve 10, an inverse pneumatic signal is produced, ie the pressure output from the valve 10 falls with increasing demand, This causes the spring brakes 12 to be applied since, in the normal running mode (no braking) the springs are held off by compressed air.
As evident from FIG. 1, the layout and construction of the conventional parking brake system requires the use of bulky spring actuators, a parking reservoir and associated pipework. All of these components require fitting and service which all adds to labour and material costs at the vehicle builders. Similar bulky spring actuators are usually also used in conventional pneumatic/mechanical (non-EBS) systems.
In accordance with the present invention, in a vehicle brake having an operating member by way of which a foundation braking force can be transmitted to a brake actuator in accordance with a driver""s intentions, there is provided a park brake mechanism which provides the facility of electric motor driven actuation of the operating member of the brake for the parking phase of braking.
Preferably, the park brake mechanism comprises an electric-motor driven actuator, such as a cam surface or a wedge, whose displacement by the electric motor is arranged to cause sympathetic displacement of said operating member for applying/removing parking braking.
Advantageously, said electric-motor driven actuator engages directly with the operating member for displacing the operating member to achieve parking braking.
Preferably, in the case of the use of a cam surface, the cam surface is selectively rotatable by the electric motor and, in the case of the use of a wedge, the wedge is selectively linearly displaceable by the electric motor for displacing the operating member of the brake.
It will be appreciated that the use of the electric-motor driven actuator, such as the wedge or cam surface, enables the operating member to be driven from a condition of brakes-off up to a condition of fully parked load without the requirement of the foundation actuation system to firstly apply the brakes, ie the operation of the parking brake function can be independent of the foundation braking condition.
Advantageously, the cam surface or wedge is displaced by the electric motor via a gearbox.
By use of the present system, a park brake demand can be arranged to be applied at a steady rate over part of one or more seconds. Using a small, high speed motor and a suitable reduction gearbox which amplifies the torque applied, the wedge or cam in use for moving the operating member of the brake can move from zero to a prescribed park load within acceptable time limits. Once there, the power is removed from the actuator and by virtue of driving through a non-reversible drive, the brake becomes mechanically latched at the prescribed level. Release of the brake is simply a reversal of the application, down to a zero or clearance park condition.
As mentioned hereinbefore, the present system is not limited to uses within EBS systems but is equally applicable to conventional brake actuation systems fitted with parking signalling to the park latch system.
The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic illustration of an example of a conventional parking brake system in an EBS system;
FIG. 2 is a diagrammatic illustration of an EBS system within which a park braking mechanism in accordance with the present invention can be incorporated;
FIG. 3 is a diagrammatic illustration of a first embodiment of a park brake mechanism in accordance with the present invention;
FIG. 4 is a sectional view through a braking device fitted with a second embodiment of a park braking mechanism in accordance with the present invention
FIG. 5 illustrates the principle of operation of the park braking mechanism of FIG. 4; and
FIGS. 6 and 7 are diagrammatic illustrations of two means of introducing compliance in embodiments using a wedge.